GEOGNOSY AND GEOLOGY. 129 
frequently have a somewhat definite structure, variously colored layers, 
conformable to the walls of the cavity, as in pl. 48, fig. 19, or lying 
horizontally, as in figs. 17 and 18, alternating with each other, and not 
untrequently leaving cavities at the centre, which become filled with 
crysiallizations of various mineral substances. The same thing occurs in 
amygdaloid (fig. 16), a rock containing spherical, a, ellipso-spheroidal, d, 
almond-shaped, 0b, cavities, filled with calcareous spar, these varying 
between the size of a few lines and several inches. Stalactitic formations are 
also met with in such cavities, as in fig. 17. The contents have most 
probably been introduced by infiltration. It is well known that water 
containing carbonic acid gas in solution, can dissolve carbonate of lime, of 
which calcareous spar consists. This solution, penetrating into the cavity, 
there deposits its mineral matter. It may be asked whence comes the 
carbonic acid of the water? The question is not difficult to answer when 
we recollect that a small proportion, about one volume in 2000, of our 
atmosphere, consists of carbonic acid gas, which, being dissolved by the 
descending rain, is brought into contact with calcareous rocks. That 
water is capable of penetrating solid rocks, is well shown by the amount 
contained in stone dug out of the ground, unless these be of a glassy 
character, as is the case with obsidian, pitch-stone, &c. The infiltration of 
water through the solid roofs of caverns is another instance. Spaces nearly 
or entirely filled up, sometimes clearly exhibit the point at which the water 
penetrated. The continuity of the layers is seen to be interrupted at this 
place, as in figs. 18 and 19; @ represents the layers which leave an open 
tube at d, through which the infiltration took place. Rocks and mineral 
substances occur in such spaces as subordinate beds of the most different 
extent and character. The character depends upon the accompanying 
minerals, some of which, as iron pyrites, copper pyrites, various iron ores, 
&c., may be advantageously worked by the miner. Small cavities of the 
kind are called nests. They are either isolated or connected, and not 
unfrequently stand in such relation to the associated rocks, as to form a 
continuous succession of beds parallel to the latter. 
15. Veins. 
Of the subordinate members of various groups of rocks, those deserve 
particular mention which, while exhibiting a great preponderance of one 
dimension over the other two, stand in no connexion with the structure of 
the rock itself. Veins are of this character: they belong to the most 
important forms exhibited by the earth’s crust, since it is from them that 
most of the metallic ores and native metals are derived; their interest, 
however, does not depend entirely on the fact of their containing these 
useful or valuable substances, but also on the mineralogical beauty of 
crystallization and form frequently presented by their contents. Veins 
break through and traverse the stratified and amorphous rocks at various 
angles, rarely following the lines of stratification or cleavage, and then anly 
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